1. Field of the Invention
The present invention relates to fuel cell monitoring devices for monitoring various operational conditions of a fuel cell comprised of one or more unit cells.
2. Description of the Related Art
There have been known a conventional technique to detect a water content in a fuel cell on the basis of impedances of the fuel cell at different frequencies, and adjust the water content in the fuel cell to an optimum value. For example, Japanese patent laid open publication No. JP 2003-86220 discloses such a conventional technique.
The technique disclosed in JP 2003-86220 calculates a resistance component R1 and a resistance component R2, and estimates a water content of the fuel cell on the basis of the resistance component R1 and the resistance component R2, where the resistance component R1 is increased when the water content of the fuel cell is insufficient (in a dried-up state), and the resistance component R2 is increased when an excessive water content is present in the fuel cell (in a flooding state). The resistance component R1 corresponds to a proton transfer resistance in an electrolyte film in the fuel cell. The resistance component R2 corresponds to a gas reaction resistance which is obtained by converting an activation overvoltage and a concentration overvoltage (a diffusion overvoltage).
The conventional method previously described calculates the gas reaction resistance, but the calculated gas reaction resistance is significantly affected by various conditions, for example, a condition when diffusion of a reaction gas is blocked in the inside of the fuel cell if the fuel cell is flooded. In addition, the calculated gas reaction resistance is significantly affected by a change of a gas concentration generated when a gas supply amount is changed.
Accordingly, the conventional technique for detecting a water content in the inside of the fuel cell has an insufficient detection accuracy because of calculating the water content of the fuel cell on the basis of the gas reaction resistance only. In other words, it is difficult for the conventional techniques to detect a correct excess amount of water in the inside of the fuel cell with a high accuracy.
In general, a diffusion state of a reaction gas such as an oxidation gas and a fuel gas is decreased or deteriorated in the inside of the fuel cell when flooding occurs in the inside of the fuel cell. It is possible to detect a correct excess amount of water contained in the inside of the fuel cell on the basis of a gas diffusion resistance because the gas diffusion resistance clearly shows a degree of difficulty of diffusion of the reaction gas in the inside of the fuel cell.
The inventors of the present invention have known that such a gas diffusion resistance in a fuel cell can be quantitatively detected on the basis of a gas reaction resistance and a limit current density. By the way, the gas diffusion resistance often varies when the dried-up state is generated in the inside of the fuel cell.
For example, when a resistance component in a catalyst layer in the fuel cell increases by the dried-up state in the inside of the fuel cell, this causes a difficulty for the reaction gas to reach the catalyst layer, and the gas diffusion resistance thereby increases. That is, the gas diffusion resistance has diffusion characteristics depending on the dried-up state in the inside of the fuel cell in addition to the flooding state in the inside of the fuel cell.
Accordingly, even if a water content in the inside of the fuel cell is normally detected on the basis of the gas diffusion resistance only, it is difficult to detect a correct excess amount of water contained in the inside of the fuel cell.